Method and device for cutting flat work pieces made of a brittle material

ABSTRACT

The apparatus for cutting a flat glass work piece along a curved dividing line includes a laser generating a laser beam having a linear beam profile and an optical system with a scanner for producing a curvilinear focal point from the laser beam. The apparatus also moves a cold spot along the curved dividing line following the curvilinear focal point. A trajectory control device controls the position of the curvilinear focal point on the curved dividing line via actuators. A profile control device controls the contour of the curvilinear focal point according to trajectory data from the curved dividing line, so that all points of the curvilinear focal point lie on or coincide with the curved dividing line. The apparatus adjusts the length of the curvilinear focal point by adjusting scanning amplitude of the scanning motion.

The invention is based on a method for cutting flat work pieces made ofa brittle material, in particular glass or ceramic, in which a laserbeam having a linear beam profile followed by a cold spot is moved alonga dividing line having a specified contour. A preferred application ofthe method is in the cutting of flat glasses.

The invention is based further on a device for cutting a work piece ofthis type using a laser beam in the form of a linear beam profilefollowed by a cold spot.

Conventional methods for cutting flat glasses are based on the use of adiamond or a small rotary cutter to first produce a scribed line in theglass in order to then break the glass by application of an externalmechanical force along the weak point produced in this fashion (“laserscribe and break”). The disadvantage of this method is that the scribedline causes particles (fragments) to be released from the surface, whichsaid particles can deposit on the glass and scratch it, for example.Additionally, “chips” can be created in the cut edge, which results inan uneven glass edge. Furthermore, the micro-cracks produced in the cutedge during the scribing process lead to reduced mechanicalstressability, i.e., to increased risk of breakage.

An approach for preventing the formation of fragments as well as chipsand micro-cracks is to cut glass based-on thermally induced mechanicaltension. In this approach, a heat source directed at the glass is movedat a fixed speed relative to the glass, thereby producing such a highthermal mechanical tension that cracks form in the glass. Infraredemitters, special gas burners and lasers, in particular, possess thenecessary property of the heat source to position the thermal energylocally, i.e., with an accuracy of greater than one millimeter, whichcorresponds to typical cutting accuracies. Lasers have proven to beeffective and have gained acceptance due to their good focusability,good controllability of output, and the ability to shape the beam and,therefore, to distribute intensity on glass. As a result, the glass canfirst be scribed using the laser beam and then broken mechanically. Or,the glass can be separated directly using the beam in conjunction with amechanically-applied starting fissure, i.e., it can be cut. The terms“separate/divide” or “cut” are intended to encompass the terms“scribe-break” as well as “cut”.

This laser beam separating method—which induces a thermal mechanicaltension up to above the breaking strength of the material by means oflocal heating using the focused laser beam in conjunction with coolingapplied from the outside—has been made known in numerous publications,e.g., in EP 0 872 303 A 2.

The laser beam separating methods mentioned hereinabove differ inparticular by the configuration of the focal point. The method accordingto DE 693 04 194 T 2, for example, uses a laser beam having anelliptical cross section followed by a cold spot.

The publication EP 0 872 303 A 2 cited hereinabove describes a laserbeam separating method that provides a focal point having a U-shaped orV-shaped contour that opens in the direction of separation. Contoursderived from these, such as X-shaped focal points, are also described.In both cases, the laser beam focal points have a two-dimensionalstructure that has proven effective in accomplishing straight cuts. Whenmaking freeform cuts, a curved, two-dimensional focal point adapted tothe contour of the dividing line would have to be produced and movedalong the contour, including the cooling that follows said focal point.This would require, in particular, coupling the scanner device producingthe respective two-dimensional focal point plus the cold spot device toa trajectory control device, the realization of which is veryproblematic due to the large quantities of data to be exchanged and thecutting speeds required.

A laser beam separating method has been made known in DE 43 05 107 C 2in which the laser beam is shaped statically, i.e., using stationaryoptical components, in such a fashion that its beam cross-section has alinear shape on the surface of the work piece, and in which the ratio oflength and width of the impinging beam cross-section can be adjustedusing an aperture in the laser beam path. This method is greatlyrestricted as well in terms of its usability. Due to the staticgeneration of the linear focal opint, freeform cuts cannot beaccomplished and, because the cooling is not to be applied until afterthe dividing line has been heated completely, e.g., using a jet of coldcompressed air, the known method is suited practically only for use asdescribed to cut off the extruded rim of hollow glassware, in whichmethod the hollow glassware rotates in the stationary laser beam,whereby the rim is first heated all the way around its circumference bymeans of the laser beam and then cooled in supportive fashion by blowingoff the gas.

The invention is based on the object of carrying out the methoddescribed initially in such a fashion, and of designing the associateddevice in such a fashion that freeform cuts can be accomplishedrelatively easily using the laser beam separating method.

Based on the method for cutting flat work pieces made of a brittlematerial, in which a laser beam having a linear beam profile followed bycold spot is moved along a dividing line having a specified contour, theobject is attained according to the method by the fact that a linearfocal point is produced on the work piece by scanning the laser beam,and trajectory data from the dividing line are made available duringevery scanning motion such that the linear focal point undergoescurvature corresponding to the curvature of the contour of the dividingline by adjusting the scanning amplitude.

As a result of these measures, it is possible to accomplish freeformcuts having any shape, because the linear focal point—in contrast-totwo-dimensional focal points with manageable quantities of data—can beadjusted in terms of its curvature and, in fact, in accordance with thecontour of the dividing line, so that the focal point is kept on thisdividing line while the length of the linear focal point is adapted tothe curvature of the contour, so that, when small radii of curvature areinvolved, the length of the focal point is correspondingly short and,when large radii are involved, the length of the focal point iscorrespondingly long, in order to ensure the necessary application ofenergy to the dividing line.

To prevent the work piece material from melting, the method is carriedout according to one embodiment of the invention in such a fashion thatthe laser output is adjusted as a function of the length of the linearfocal point.

The generation of an optimal thermal mechanical tension in the workpiece can be achieved when the cold spot is moved along behind thelinear focal point in such a fashion that its distance from the startingpoint of the focal point is constant.

With regard for the device for carrying out the method according to theinvention, the object is attained by the fact that an optical systemhaving a scanner for producing a linear focal point is provided, whichsaid optical system is coupled to a numerical trajectory control devicevia a profile control device that specifies the contour of the focalpoint by means of axes U and V and via actuators that specifypositioning axes X, Y, C of the focal point in relation to the workpiece such that trajectory data from the dividing line can be forwardedto the profile control device and the actuators during every scanningmotion such that the linear focal point undergoes curvaturecorresponding to the curvature of the contour of the dividing line andis moveable along said dividing line, and the length of which saidlinear focal point is also adjustable as a function of the contour ofthe dividing line.

An optimal separation is ensured when, according to one embodiment ofthe invention, further axes coupled to the trajectory control device areprovided to position the cold spot in relation to the linear focal pointand the work piece.

According to a further embodiment of the invention, the device can bedesigned particularly suitable in nature when the optical systemcomprises a scanner having two oscillating mirrors oscillating at rightangles to each other. To change the length and curvature of the linearfocal point, one only need to change the oscillation amplitude of theoscillating mirror, so that the profile control device and thetrajectory control device can be designed relatively simple in nature aswell. Not only is the oscillation amplitude changed in this process, butso is the mode of motion and the position of the oscillations relativeto each other. The oscillations are not harmonic in transitions betweencurves and straight lines, for example.

Further features and advantages of the invention result from thedescription of an exemplary embodiment shown in the drawing.

FIG. 1 shows a schematic block diagram of the basic layout of thecontour controller of the device according to the invention,

FIG. 2 shows a schematic, diagram-like illustration of the shape of thefocal point and the cold spot, as well as the associated axes of motionto be worked by the contour controller, and

FIG. 3 shows a schematic drawing of the length of the linear focal pointthat differs as a function of the curvature of the contour of thedividing line in the case of a special freeform cut.

The principle according to the invention for cutting a flat work piecemade of a brittle material, in particular flat glasses, using the laserbeam separating method is shown in FIGS. 1 and 2. FIG. 1 shows theprinciple layout of the control of the sequences of motions along aspecified contour. FIG. 2 shows the focal point geometry of the laserbeam with the cold spot following it, and the matching of the axes ofmotion to the contour controller according to FIG. 1.

The separating principle according to the invention is based on a linearfocal point 1 that is curved in accordance with the respective contourof the dividing line 2. If the dividing line 2 is linear, the focal line1 can be relatively long, as shown in FIG. 3. The smaller the radius ofcurvature of the contour of the dividing line 2 is, the shorter the line1 must be, so that the main application of energy takes place on thedividing line. The line length varies hereby between 10 mm and 100 mm.In parallel with changing the length of the line, the laser output isregulated suitably downward when short lines are involved, and it isregulated upward when long lines are involved.

To produce the linear focal point 1, a known laser-beam scanner havingoscillating mirrors and an oscillator motor that moves them, preferablyin the form of a galvanometer mirror, e.g., according to the FIG. 5 ofthe publication EP 0 872 303 A 2 cited initially, is preferably used. Inprinciple, just one mirror could also be provided that is supported in afashion that allows it to pivot.

In addition, a scanning procedure can be used that uses a rotatingmirror according to FIG. 6 of the aforementioned EP publication(polygonal wheel).

In the typical scanning process using oscillating mirrors, saidoscillating mirrors have an oscillation frequency of 400 Hz and higher,whereby the oscillation amplitude of a first oscillating mirrordetermines the line length, and the oscillation amplitude of the other,second oscillating mirror determines the deviation of the line from astraight line. The scanner is controlled by means of a profile controldevice 3 shown in FIG. 1. This control device receives—from a numericalcontrol device—the desired value, among other things, for the length ofthe focal point 1 required at the respective point of the dividing line2. The profile control device thereby drives the oscillator motor of thefirst oscillating mirror in such a fashion that it executes theappropriate oscillation amplitude for the required length of theprofile. In this process, the profile control device 3 generates ananalog voltage fluctuation as the desired value, which said desiredvalue is converted directly by the oscillator motor into an oscillatingmotion. The desired value—which is forwarded by the numerical controldevice 4 to the profile control device 3—is transmitted either in theform of an analog constant voltage signal, e.g., 0-10 V, whereby thelength is proportional to the voltage, or in the form of a digital valuewith a resolution of at least 8 bits. The actual-value feedback by themeasuring system of the oscillator motor shown in the illustration canbe eliminated within the scope of regulation if a calibration curve forthe oscillator motor is stored in the profile control device 3 or thenumerical control device 4. In addition to the specification of theoscillation amplitude, the scanner receives from the numerical controldevice 4 data for the axes U, V during each oscillation of the mirror atspecified instants—indicated in FIG. 2 using an “X”—which said datacontrol the second oscillating mirror in such a fashion that the focalline 1 is curved in accordance with the contour of the dividing line 2during the respective oscillation of the first mirror. The more datathat can be provided per oscillation, the better the curvature can beapproximated.

To ensure that the focal line 1 lies on the desired dividing line 2, thenumerical control device 4 controls the NC axes 8 (X, Y and C) and theprofile length or its curvature by communicating the desired values ofthe axes U and V to the profile control device 3 according to a known NCprogram 5. Using the information about the profile length, its curvatureby means of the values for the axes U and V, the position and speeds ofthe axes X, Y and C, and the shape of the dividing line 2 described byan NC program 5, the numerical control device is capable of guiding thefocal line 1 in such a fashion that it follows the dividing line with amaximum deviation of typically less than 0.2 mm. The dividing line isfollowed that much more exactly the faster the numerical control deviceis capable of generating new desired values for the axes 8 and theprofile control device 3, and the faster these said desired values canbe implemented by the axes 8 and the oscillator motors of theoscillating mirror.

FIG. 3 shows an example of a freeform cut having a trapezoidal contourof the dividing line 2 with sharply rounded corners.

The focal point 1 is linear at instant t₁, because a linear section isto be separated. At instant t₂, the focal point softens a sharply curvedsection of the dividing line. It is then curved and shortened based onthe data from the trajectory control device in accordance with thedividing line.

At instant t₃, the focal point 1 is situated on a less curved section ofthe separating line 2. As a result, it is less curved and also somewhatlonger than at instant t₂. At instant t₄, the focal point 1 is againsituated in a linear section, and it has the same dimensions that it didin instant t₁.

The width of the focal line 1 is specified by the diameter of thefocused laser beam on the work piece surface. The diameter on the workpiece surface is typically between 0.3 mm and many millimeters. Opticalelements are used to focus the laser beam. In selecting the opticalelements and the laser, the objective is to distribute the intensityevenly along the line without any local intensity peaks. In this manner,the greater part of the laser energy can be injected into the work piecewithout exceeding the glass softening temperature. For this purpose,focusing elements having a long focal length of approximately 300 mm andabove, top-hat lenses, axicons or lasers having a ring mode ormulti-mode can be used. The width of the line depends thereby on therequired laser output, cooling, material type, material thickness andfeed rate.

A linear profile of any length and shape can be provided using themethod according to the invention.

To increase the thermal mechanical tension, cooling is carried out inknown fashion using a cold spot 6 situated at a defined distance behindthe laser profile, i.e., the focal line 1, on the contour 2 to be cut.This cold spot 6 is produced, for example, by means of a cold jet of airor a gas/liquid mixture injected via a nozzle.

Due to the high temperature produced in the work piece by means of thelaser beam, a high thermal mechanical tension is created along thecontour to be cut in the work piece. When cooling is appliedsubsequently and the glass is weakened in advance at the starting pointof the cut/fissure, the glass cracks along the contour described by thecooling nozzle and the “beam line”.

Due to the heating of the work piece achieved along the contour to becut and the cooling carried out following it on the contour at adefining distance of between approximately 2 mm and 15 mm, the cutfollows any possible free form very precisely. In this fashion, anypossible geometry can be cut in thin glass (approx. 50 μm), as well asthick glass (many millimeters), or scribing can be carried out at adepth of up to many tenths of millimeters.

The advantage of cutting lies in the fact that subsequent breaking isnot required, which eliminates a finishing step. The advantage ofscribing lies in the fact that, at much higher speeds (up to 1000 mm/s),the material was separated nearly fragment-free even after breaking and,due to the lack of micro-cracks and chips, a markedly higher edgerigidity is obtained.

In order to achieve this, the cooling, i.e., the position of the coldspot 6 relative to the focal line 1, must also be adjusted veryprecisely.

The axes 7 (A and B) shown in FIG. 1—which are responsible for thetrailing guidance of the cooling nozzle along the dividing line 2—arealso connected to the numerical control device 4. The axes A and B alsointerpolate with the axes X, Y, C and the axis specified by theoscillator motor of the first oscillating mirror.

The object of the axes A and B is to maintain a constant distancebetween the cold spot 6 and the starting point of the focal line 1. Theprofile control device 3 can hereby be designed so that the lowestvoltage value of the fluctuating voltage for the oscillator of theoscillating mirror always remains constant. This ensures that thisstarting point is stationary. The axes 7 A and B therefore need onlycover short distances.

To simplify the construction of the system, the axis A can be eliminatedin one variant of the implementation. In this fashion, a non-constantdistance between the cooling and the starting position of the focal lineresults when the contour is followed. This situation can be offset whilereducing the cutting speed by adapting the process parameters.

“Axes” in the context of the invention should be understood to mean notonly the geometric axes, but also the associated moving members, such asactuators and the like, that specify the axes.

1-6. (canceled).
 7. A device for carrying out a method for cutting flatwork pieces made of a brittle material, said method comprising moving alaser beam having a linear beam profile followed by cold spot along adividing line having a specified contour, wherein a linear focal pointis produced on the work piece by scanning the laser beam, and thescanning of the laser beam is controlled according to trajectory dataregarding the dividing line made available during every scanning motionso that all points of the linear focal point lie on or coincide with thedividing line, and a length of said linear focal point is adjusted as afunction of curvature of the contour of the dividing line by adjusting ascanning amplitude; wherein said device comprises an optical systemhaving a scanner for producing the linear focal point and said opticalsystem is coupled to a numerical trajectory control device via a profilecontrol device that specifies a shape of the linear focal point by meansof axes U and V, and via actuators that specify positioning axes X, Y, Cof the linear focal point in relation to the work piece, such that thetrajectory data regarding the dividing line is forwarded to the profilecontrol device and the actuators during every scanning motion so thatsaid points of said linear focal point lie on or coincide with thedividing line and said linear focal point is moveable along saiddividing line, and the length of said linear focal point is adjustableas a function of the specified contour of the dividing line.
 8. Thedevice according to claim 7, further comprising means for positioningthe cold spot in relation to the linear focal point and the work pieceand wherein said means for positioning the cold spot are coupled withthe trajectory control device.
 9. The device according to claim 7,wherein the scanner has two oscillating mirrors oscillating at rightangles to each other for positioning the linear focal point on thedividing line.
 10. A device for cutting flat work pieces made of abrittle material, said device comprising: means for moving a laser beamhaving a linear beam profile along a dividing line on a work piece, saiddividing line having a specific contour; means for moving a cold spotalong said dividing line following said laser beam; means for scanningsaid laser beam in an oscillatory manner during the moving of the laserbeam, so that a linear focal point is produced on the work piece, saidmeans for scanning comprising an optical system with a scanner; meansfor providing trajectory data regarding a course of the dividing linefrom a numerical trajectory control device to a profile control devicethat specifies a curvature of the linear focal point and to an actuatingmeans that specifies a position of the linear focal point on thedividing line; means for controlling the curvature of the linear focalpoint with the profile control device during the scanning so that allpoints of the linear focal point lie on or coincide with the dividingline; and means for adjusting a scanning amplitude so as to adjust alength of said linear focal point as a function of the specified contourof the dividing line.
 11. The device according to claim 10, wherein saidscanner has two oscillating mirrors oscillating at right angles to eachother for positioning the linear focal point on the dividing line. 12.The device according to claim 10, further comprising means forpositioning and guiding the cold spot in relation to the linear focalpoint.